1. Field of the Invention
The present invention relates to high-capacity optical disc techniques, and more particularly to a reproduction method for determining an optimum light power to reproduce information recorded on a high-capacity optical disc, a reproduction device that determines an optimum light power to reproduce information, and further an information recording medium in which an optimum light power is recorded.
2. Description of the Related Art
Up to now, as the large capacity information recording technology, high density optical recording techniques has been studied and developed to allow more information to be stored in a unit area. In the technique of optical discs currently commercialized, a laser beam is focused on a disc surface to record and/or reproduce data recorded on the disc. In order to increase the density of data, a technique for making the size of a focused laser spot smaller has been developed so far. It is known that the spot size is proportional to λ/NA, where λ and NA denotes the wavelength of a light source and the numerical aperture of an objective lens, respectively. That is to say, the amount of information stored in one disc has been increased by shortening the wavelength of a light source and increasing NA of a lens. Here, if a set of the wavelength of a light source, NA of an objective lens, and the capacity of data stored in a disc of a 12 cm diameter is denoted as (wavelength, NA, capacity), then it is (780 nm, 0.5, 650 MB) in CD and (650 nm, 0.6, 4.7 GB) in DVD. Moreover, two kinds of sets (405 nm, 0.85, 25 GB) and (405 nm, 0.65, 20 GB) have been proposed for the techniques using a blue laser light source. With this storage capacity, it is possible to record high definition TV image data for about two hours.
However, the above-described storage capacity is insufficient, for instance, for use in business systems, such as broadcasting stations, and in security systems, where, for instance, one disc is required to have a capacity of no less than 100 GB. Moreover, there is a demand for a disc for storing image data or the like desired to be stored for a long period of time as long as several tens of years to about 100 years. In this case, a single disc is desired to store as much data as possible in order to save the storage place for media storing such a great amount of data. The required capacity is from several hundred GB to 1 TB or more.
However, achieving a further increase in the capacity using the above-described method is difficult. First of all, it is extremely difficult to develop a semiconductor laser as a light source with a further shorter wavelength. Moreover, even if a laser diode were developed, securing excellent record/reproduction quality is expected to be difficult since the disc substrate and protective film would absorb light because the light source emits ultraviolet light. Research on increasing NA of an objective lens is currently underway, and a technique in the case where NA is set to 1.8 is reported in Japanese Journal of Applied Physics, Vol. 42, pp. 1101-1104, for example. However, in this system, the light used in recording/reproducing is not a regular propagating light but a so-called near-field light, which is a light localized at the lens, so that the system needs to have a mechanism of moving a lens above a disc while bringing the lens extremely close to the disc surface and maintaining the distance between them. Such system is similar to a hard disk technology for magnetic recording, and thus has a disadvantage of the difficulty of replacing discs, which is an advantage of the optical disc.
According to the background described above, a method for effectively improving the optical resolution by providing some mechanism in a disc has been proposed. Here, this method will be referred to as a super-resolution technique.
A super-resolution technique using a phase change recording film is reported in Japanese Journal of Applied Physics, Vol. 32, pp. 5210-5213. The phase change recording film is typically used as a recording film for a rewritable disc such as CD-RW, DVD-RAM, DVD±RW, and a Blu-ray Disc, or the like. However, here, this recording material is not used as the recording film but used as a layer to effectively improve the optical resolution like the reproducing layer in the above-described optical magnetic disc. Herein, such layer (film) will be referred to as a super-resolution layer (film). In this case, the data recorded on the disc is recorded not on the here-termed super-resolution layer, but on another place. For example, in the case of a read-only (ROM) disc, the data is recorded in the form of concavity or convexity on the substrate, and in the case of a recordable-type disc, a recording film is provided in addition to the here-termed super-resolution layer, and the data will be recorded on this recording film. As a typical example of this technique, the super-resolution layer and a layer on which data is recorded are provided in a similar fashion within the depth of focus of a beam to be irradiated, where the interlayer distance is several tens to several hundreds nm. In this technique, the phase change recording film is deposited on a read-only (ROM) disc by sputtering and a part of the phase change recording film is melted during reproduction. If the reflectivity of the disc at the melted portion is sufficiently high, the signal obtained from the melted portion becomes dominant in the reproduction signals. That is, the melted portion of the phase change film serves as the effective reproducing light spot. This means that the reproducing light spot has been reduced because the area of the melted portion is smaller than the light spot, so that the optical resolution is improved.
Japanese Patent Application Publication No. 2006-107588 has further developed this approach and proposed a method, in which pits made of a phase change material is prepared and then a single pit is melted during reproduction to thereby obtain a super-resolution effect. In this proposal, the pits are made of a phase change material by using a phase change etching technique. The phase change etching technique is a technique of transforming the patterns of phase change marks into concaves and convexities by utilizing a difference in solubility to an alkaline solution between the crystalline part and amorphous part of the phase change film. In this technique, since a material exhibiting the super-resolution effect is present only in the mark portion, the space portion does not absorb light at all. Accordingly, the optical transmissivity of one layer can be improved to allow for a combination of the multilayer technique and the super-resolution technique. An example achieving a dual-layer super-resolution disc with this technique is reported in Japanese Journal of Applied Physics, Vol. 45, pp. 2593-2597. This method will be referred to as a pit type super-resolution scheme, and the case where a super-resolution thin film is successively deposited in two dimensions as described above will be referred to as a thin film type super-resolution scheme.
Furthermore, a method for suppressing a normal-resolution crosstalk, which is a problem common to super-resolution techniques, using the pit type super-resolution scheme described above may be contemplated. The normal-resolution crosstalk is described below. Usually, in the super-resolution scheme, as described above, the optical resolution is improved using a high temperature area formed near the center of a light spot as the effective spot. In this case, actually, a low temperature area is also irradiated with light, so that a light signal reflected at the medium and entering a photo detector includes also an influence from recording marks existing within the low temperature area. In the super-resolution scheme, the desirable signal is the light signal obtained only from the high temperature area, and this signal will be referred to as a super-resolution signal. The signal from the low temperature area has a different frequency characteristic from that of the super-resolution signal and causes a random influence on this super-resolution signal in reproducing a random data sequence. Accordingly, the signal from low temperature area is an element disturbing the reproducing signal. Here, a signal generated from this low temperature area is referred to as the normal-resolution crosstalk.
This normal-resolution crosstalk can be suppressed using a method shown below, for example. Herein, a ROM disc is described as an example. In the final form of the disc, a super-resolution film is embedded only in a recording mark, which is a concave portion in the disc substrate, as described in Japanese Journal of Applied Physics, Vol. 42, pp. 1101-1104. This form is obtained by chemically mechanically polishing the disc after the deposition. Here, the complex reflectivities of a mark portion and a space portion are made equal in the design of the film stack of the disc. The mark and space have thus completely the same optical characteristic with respect to an incident laser beam. Accordingly, even if a mark is present, a signal cannot be obtained in a drive (information reproducing device) but if the optical characteristic of the mark portion changes due to a super-resolution phenomenon, a reproduction signal will be obtained. Namely, since the reproduction signal is obtained only after the super-resolution phenomenon occurs, the normal-resolution crosstalk can be suppressed consequently. This suppresses the normal-resolution crosstalk both between tracks and within a track, so that the track density can be also improved.
The method for obtaining a tracking error signal for carrying out tracking servo may be an issue in this case. For example, in a push-pull scheme, which is a currently widely used tracking method, an intensity difference (push-pull signal) in intensity between diffracted light beams generated on right and left sides of the traveling direction of a spot is detected at the edge of a track groove or a pit. If the difference is finite, then it is determined that the track center and the spot center deviate from each other, and the spot position is corrected. In this method, when the track density is increased, the diffraction efficiency will decrease, which thereby reduces the quantity of a push-pull signal described above and reduces the signal-noise ratio (SNR) of the push-pull signal. This causes a tracking error.
This problem is solved by carrying out tracking servo using a medium having normal-resolution crosstalk suppressed as described above and using a push-pull signal caused by super-resolution conditions. From the above-described medium having the suppressed normal-resolution crosstalk, a push-pull signal cannot be obtained in non-super-resolution conditions. However, if there is an optical phase difference between a mark portion and a space portion in super-resolution conditions, a push-pull signal can be obtained. Furthermore, in this case, since a push-pull signal can be obtained from only one mark, a problem of a decrease in the diffraction efficiency will not occur even if the track pitch is small.
In order to obtain a super-resolution and high-quality reproduction signal, the super-resolution area is preferably in size between the shortest mark length and the second shortest mark length. Here, the second shortest mark length represents the length of a mark having the second shortest length among digitized marks recorded on the medium. The size of the super-resolution area can be controlled through adjustment of the reproduction laser power. This is because an increase in the reproduction laser power will increase the size of a high temperature super-resolution area while a decrease in the power will reduce the size of the super-resolution area, for example. The method for adjusting the reproduction laser power is described in Japanese Patent Application Publication No. 2006-107588, for example. In this method, a mark having a predetermined pattern is recorded on a medium in advance, and this mark is reproduced by means of super-resolution, whereby the reproduction power at which the bit error rate of the reproduction signal becomes the minimum is determined.